My suggestion is that when you're copying stuff, always go for the better quality setting.
When you're recording (not taping...hard habit to break, isn't it? :iagree: ), use the SP setting, if you don't want to use the HQ one hour setting.
To go with the four hour setting could be a mistake, since some have had occasional problems with the processor dropping frames and creating a strobe-like effect for a minute or two here and there.
If you have the three hour option on your machine, that might be a bit less likely to give you the problem. That way you can use two discs per tape.
Now, if it wasn't for the strobe/lost frame problem, I'd say go with four hour, because the picture quality itself is very good.
The odd thing is, while my machine did have the strobe problem to begin with, I've occasionally used the three and four hour mode anyway and have found that for some reason, with time, the problem has shown up less and less.
Just food for thought.
Oh, and as for "what speed's best for what speed" I don't think that's really a factor. Just go with what will preserve the picture quality the best. The respective speeds of the original and the copy really (in itself) has no effect.
Just go with what preserves quality.